1. Field of the Invention
The present invention relates generally to carburetion systems for natural gas fueled internal combustion engines; and, more particularly, to venturi carburetion systems for natural gas fueled internal combustion engines using recycled exhaust gas.
2. Description of Related Art
Electric energy generation in this country has lagged behind demand. There are a number of reasons for this, but chief among them is failure of traditional energy producers to replace spent units and capitalize new plants. This has been, in part, due to increased air quality regulations. In addition, new challenges face electric generation security. Events of Sep. 11, 2001 showed this nation its vulnerability to terrorist attack. Vital operations, such as police, medical and civil defense that relied upon the electric power “grid” for service, realized that their needs were susceptible to disruption and viewed stand-alone units, as well as micro grids as a possible solution. These alternatives are fraught with their own problems. Chief among the reasons is a drastic increase in demand. Thus, while energy demand has increased, generating capabilities have not.
In addition to the mismatch between demand and generating capacity, the physical transmission infrastructure necessary to deliver power from geographically remote generating facilities to the consumer's location is unable to support the increased load. Even under today's operating conditions, the transmission grid is subject to stress and occasional failure.
One reason for the growth in demand is the increased use of computers and other technology for industrial and business purposes, as well as personal use. As computer usage continues to grow, the use of power-consuming peripheral technologies, such as printers, cameras, copiers, photo processors, servers, and the like, keep pace and even expand. As business use of computer based equipment continues to rise, as do the number of in-house data servers, outsourced data storage facilities, financial systems, and Internet-related companies requiring constant electrical uptime and somewhat reducing traditional peak demand times, requirement for reliable, cheap, environmentally compliant electrical power, continues to grow.
Other technological advances have also increased electrical energy demand. Increased use of power consuming devices in every aspect of life, from medical to industrial manufacturing robots, as well as innovations in almost every research and industrial field, are supported by increasingly complex technology, which requires more electrical power to function. CAT scans, NMRs, side looking X-rays, MRIs and the like, all take electrical power.
Further, security and reliability of source has become of increasing concern. Grid system vulnerability is a real threat. Strategic industries are looking to cut energy costs, increase reliability, and assure security. This has lead to an interest in distributed market technologies. The potential market for distributed generation has become vast without adequate means for fulfilling this need. Again, inefficiency, reliability, and environmental concerns are major barriers. The compelling economics are made on engine efficiency without the financial benefit of waste heat usage, yet with all of the same customer reluctance to accept hassles. Industry estimates indicate that the existing market for distributed generation is $300 billion in the United States and $800 billion worldwide.
Although most existing distributed generation sites use small gas turbine or reciprocating engines for generation, there are many alternatives that are being considered over the longer term. Technologies, such as micro-turbines, are currently available, but only used at a relatively small number of sites. These newer generators offer some inherent advantages, including built-in communications capabilities. It is anticipated that fuel cells will be available in the next five years, which will provide some highly appealing, environmentally friendly options.
As it stands today however, small gas turbine and reciprocating engines comprise a substantial proportion of existing generator technology in the market and will for some time to come for a number of reasons. Engines provide the best conversion efficiency (40%), and they can operate using non-pressurized gas. Micro-turbines, on the other hand, require compressed gas and conversion efficiency is lower (approximately 30%). These latter generators tend to be used in wastewater and landfill and other specialty sites, where a conventional prime mover is unable to stand up to poor fuel quality. Therefore, for utilities to truly benefit from a distributed generation scheme over the short term, they must look to the existing generator technology to provide a sustainable and affordable solution.
Waste heat utilization or cogeneration is one way to increase overall system efficiency. In the case of most power generation, the waste heat is not used, and the economics are based largely on the cost of the electricity produced (i.e. heat rate is paramount), with little consideration for improved reliability or independence from the electric grid. The anticipated fluctuation in energy costs, reduced reliability, and increasing demand has led end users to consider maximizing efficiency through use of heat from generation of on-site generating-heat capture systems, i.e. cogeneration, or “Combined Heating and Power” (CHP).
Cogeneration of electricity and providing client service heat for space heating and/or hot water from the same unit is one solution. Cogeneration provides both electricity and usable process or utility heat from the formerly wasted energy inherent in the electricity generation. With cogeneration, two problems are solved for the price of one. In either case, distributed electricity generation systems must meet stringent local air quality standards, which are typically much tougher than EPA (nation wide) standards.
On-site cogeneration represents a potentially valuable resource for utilities by way of distributed generation. A utility can increase capacity by turning to a “host” site (e.g. industrial user) with an existing generator, and allow them to parallel with the grid and use their generator capacity to handle peak volumes as well as provide utility and space heat to the host site customers. From the utility's point of view, the key advantages to a distributed generation solution are twofold: improved system reliability and quality; and the ability to defer capital costs for a new transformer station.
For customers who can use the process/utility waste heat, the economics of cogeneration are compelling. The impediment to widespread use is reliability, convenience, and trouble free operation. Cogeneration products empower industrial and commercial entities to provide their own energy supply, thus meeting their demand requirements without relying on an increasingly inadequate public supply and infrastructure.
Unfortunately, to date, the most widespread and cost-effective technologies for producing electricity require burning hydrocarbon-based fuel. Other generating technologies are in use, including nuclear and hydroelectric energy, as well as alternative technologies, such as solar, wind, and geothermal energy. However, burning fuel remains the primary method of producing electricity. Unfortunately, the emissions associated with burning hydrocarbon fuels are generally considered damaging to the environment, and the Environmental Protection Agency has consistently tightened emissions standards for new power plants. Green house gases, as well as entrained and other combustion product pollutants, are environmental challenges faced by hydrocarbon-based units.
Of the fossil fuels, natural gas is the least environmentally harmful. Most natural gas is primarily composed of methane and combinations of Carbon Dioxide, Nitrogen, Ethane, Propane, Iso-Butane, N-Butane, Iso Pentane, N-Pentane, and Hexanes Plus. Natural gas has an extremely high octane number, approximately 130, thus allowing higher compression ratios and broad flammability limits. A problem with using natural gas is reduced power output when compared to gasoline, due mostly to the loss in volumetric efficiency with gaseous fuels. Another problem area is the emissions produced by these natural gas engines. Although, the emissions are potentially less than that of gasoline engines, these engines generally require some types of emissions controls such as exhaust gas recirculation (EGR), positive crankcase ventilation (PCV), and/or unique three-way catalyst.
Still another problem with using natural gas is the slow flame speed, which requires that the fuel be ignited substantially before top dead center (BTDC). In general, most internal combustion engines running on gasoline operate with a spark advance of approximately 35 degrees BTDC where as the same engine operating on natural gas will require an approximate advance of 50 degrees BTDC. The slower burn rate of the fuel results in reduced thermal efficiency and poor burns characteristics.
It is well known that emission reduction for natural gas engines can be accomplished by EGR to make the engines run lean. Numerous systems have been devised to recycle exhaust gas into the fuel-air induction system of an internal combustion engine for the purposes of pre-heating the air-fuel mixture to facilitate its complete combustion in the combustion zone, for re-using the unignited or partially burned portions of the fuel, which would otherwise pass to exhaust and into the atmosphere, and for reducing the oxides of nitrogen emitted from the exhaust system into the atmosphere. It has been found that approximately 15 to 20 percent of exhaust gas recycling is required at moderate engine loads to substantially reduce the nitrogen oxide content of the exhaust gases discharged in the atmosphere, that is, to below about 1,000 parts per million.
Although, the prior art systems have had the desired effect of reducing nitrogen oxides in the exhaust by reducing the maximum combustion temperature as a consequence of diluting the fuel-air mixture with recycled exhaust gases during certain operating conditions of the engine, these systems have not been commercially acceptable from the standpoints of both cost and operating efficiency and have been complicated by the accumulation of gummy deposits, which tend to clog the restricted bypass conduit. Recycling the exhaust systems have also been complicated by the desirability of reducing the recycling during conditions of engine idling when nitrogen oxide emission is a minor problem and progressively increasing wide open throttle when maximum power is required, while progressively increasing the recycling exhaust gases with increasing engine load.
In the usual hydrocarbon fuel type engine, fuel combustion can take place at about 1,200° F. The formation of nitrogen oxides does not become particularly objectionable until the combustion temperature exceeds about 2,200° F., but the usual engine combustion temperature, which increases with engine load or the rate of acceleration at any given speed frequently, rises to about 2,500° F. It is known that the recycling of at least one-twentieth and not more than one-fourth of the total exhaust gases through the engine, depending on the load or power demand, will reduce the combustion temperature to less than 2,200° F. Contaminants in the exhaust resulting from fuel additives desired for improved combustion characteristics normally exit in a gaseous state at combustion temperatures exceeding about 1,700° F., but tend to condense and leave a gummy residue that is particularly objectionable at the location of metering orifices and valve seats in the exhaust recycling or bypass conduit. The thermal nitrogen oxide emission is a direct function of combustion temperature and for that reason is less critical during engine idling when the rate of fuel combustion and the consequent combustion temperature are minimal but tends to be problematic during throttle-up and extended full speed operation.
Thus, prior art cogeneration systems employing internal combustion engines, and specifically, natural gas fueled engines have suffered from the myriad of problems including elevated head temperatures and inability to deliver large quantities of process and/or utility heat to the cogeneration client. Excessive head temperatures lead to inefficient operation and unacceptable environmental conditions, which include excessive use of fuel, as well as significant NOx production.
Some of the inherent problems with natural gas fueled engines, which utilize EGR techniques to reduce pollution, are a result of carburetion problems, which prevent them from running “lean.” Specifically, the natural gas regulators have been unable to supply natural gas to the engine throughout the load cycle while maintaining a fuel to air ratio, which does not starve the engine or alternatively run it to rich. In the former case, the engine stalls; in the later, fuel efficiency and NOx production become out of limits.
Prior art internal combustion engines operating on natural gas and used as power units to spin electric generators use various types of fuel carburetion, regulation, and introduction systems. One device for carburetion uses a diaphragm, which opens under engine vacuum, to operate a fuel-metering valve, allowing fuel to enter an air-mixing chamber, where it is mixed with air and exhaust gas for combustion. The mixture of fuel/air/recycle gas from the mixing chamber then passes through a throttle regulator, which regulates the flow of the mixture to the engine as a function of load. The diaphragm regulates fuel flow by responding to the changes in vacuum (pressure) at the intake manifold of the engine. In some configurations, an exhaust gas driven turbocharger is used to pre-compress the mixture prior to injection into the cylinder. The use of a turbocharger increases the vacuum on the carburetion unit.
These systems have inherent drawbacks. For one, the diaphragm must be several times the size of the gas inlet. For example, a three-inch diameter inlet could require a diaphragm of 15-18 square inches. For a second, when recycled exhaust gas is used the recycle gas tends to erode the diaphragm. Finally, these systems are prone to diaphragm rupture from engine backfire through the intake system. If the engine is turbocharged, the air/recycle gas/fuel mixture passes through the turbocharger, and then, preferably, through an engine intercooler to cool the compressed air/recycle gas/fuel mixture and into the engine cylinders. The combination of a large diaphragm section, mixing section, and throttle section in the carburetion unit make for a large and bulky apparatus, which must be mounted directly on the engine intake.
Therefore, a more fuel-efficient balanced venturi style fuel/air mixing unit was developed. This style of fuel introduction is now utilized on most natural gas fired, internal combustion engines because it is easy to obtain parts, assemble, and mount to the engine. One drawback is that a separate gas shut-off valve must be installed upstream of the venturi for engine shutdown. Another is flooding from pressurized fuel sources. Finally, the load variations on these systems make fuel regulation upstream of the venturi compelling.
Engines, which utilize this venturi fuel carburetion system, utilize a series of small ports of a size determined by the fuel requirement. Air is drawn into a venturi-mixing chamber by vacuum. If the fuel is under a positive pressure, the chamber floods prior to ignition, which prevents ignition of the engine. Again, a regulator is required upstream of the venturi. One type of regulator employs one or more diaphragms, which respond to variation in engine or burner vacuum. For most applications, this vacuum operated devise works reasonably well because, like a burner, the requirement for fuel is full on or full off.
Prior art EGR carburetion systems have been plagued with a myriad of problems. First, the exhaust gas is usually recycled at elevated temperatures, which increases the head temperature of the engine, unless the gas is cooled. In addition, when the engine is turbocharged, the temperature of the intake admixture is elevated even further by compression. Thermal NOx is a function of head temperatures, as previously described. Thus, elevated temperatures of intake gases contribute to thermal NOx production.
Second, carburetion systems for natural gas internal combustion engines that utilize recycled gas must mix recycled exhaust gas in proportion to ambient air in proportion to fuel to affect stoicheiometric conditions in the engine, while maintaining low thermal NOx and overall energy efficiency. In cogeneration units, where electric load automatically throttles the engine, these ratios of air-to-exhaust and gas-to-fuel can easily become out of proportion because of rapid acceleration/deceleration of the engine.
Thus, carburetion systems for internal combustion natural gas fired exhaust gas recycled engines have, heretofore, been complicated and involved electronic, dynamic feed back, control systems for regulating the fuel, as well as the recycled gas in response to changing engine demands. Since the intake mixture varies with load, too much fuel or too much recycled gas will “choke” the engine. Like wise, too little fuel will “starve” the engine and too little recycle gas will increase thermal NOx production. All of these conditions cause the engine to be unstable, inefficient, and out of emission compliance.
Therefore, it would be advantageous to have a simple carburetion system, which does not involve complicated control feed back systems, which must be maintained and tuned; eliminates need for mechanical or digital control of varying EGR flow over changing load or RPM; and/or the need for mechanically or electrically driven pumps to properly modulate EGR over changing loads. In addition, it would be advantageous to have a system, which mechanically responds to the engine load requirements by dynamically regulating the fuel, air, recycled gas ratio in the intake mixture as a function, solely of the pressure change created by acceleration/deceleration of the engine. Further, it would be advantageous if such system would operate without creating engine instability in response to almost instantaneous electrical load change. It would also be advantageous to accomplish all of the aforementioned without complicated mechanical linkage or excess parasitic power demands.
It would further be advantageous to have a cogeneration system with reduced fuel consumption, as well as NOx production, while delivering substantial heat to the process/utility heat cogeneration system. In addition, it would be advantageous to run a lean burning engine using EGR, which results, in not only a lean burn, but also reduced head temperatures, leading to reduced thermal emissions and greater efficiency.